1
2 THEMIS Observations of Unusual Bow Shock Motion
3 Attending a Transient Magnetospheric Event
4
5 G. I. Korotova
6 IZMIRAN, Troitsk, Moscow Region, 142190, Russia,
7 also at IPST/UMD, College Park, MD, 20742, USA
8
9 D. G. Sibeck
10 Code 674, NASA/GSFC, Greenbelt, MD 20771, USA
11
12 N. Omidi
13 Solana Scientific Inc., Solana Beach, CA, 92075, USA
14
15 V. Angelopoulos
16 IGPP/ESS, UCLA, Los Angeles, CA, 90095, USA
17
18
19
20
21 Abstract
22 We present a multipoint case study of solar wind and magnetospheric
23 observations during a transient magnetospheric compression at 2319 UT on October 15, 24 2008. We use high-time resolution magnetic field and plasma data from the THEMIS
25 and GOES-11/12 spacecraft to show that this transient event corresponded to an abrupt
26 rotation in the IMF orientation, a change in the location of the foreshock, and transient
27 outward bow shock motion. We employ results from a global hybrid code model to
28 reconcile the observations indicating transient inward magnetopause motion with the
29 outward bow shock motion.
30
31 1. Introduction 32 33 The interaction of interplanetary discontinuities with the Earth’s bow shock
34 and magnetopause has been the subject of intense research for many years. A host
35 of observational studies have demonstrated that both boundaries lie nearer Earth during
36 intervals of enhanced solar wind dynamic pressure (and magnetosonic Mach number)
37 [e.g., Fairfield, 1971; Shue et al., 1997; Merka et al., 2005]. Working within the
38 magnetohydrodynamic (MHD) framework, Volk and Auer [1974], Wu et al. [1993],
39 Cable and Lin [1998], and Samsonov et al. [2007] showed that the interaction of an
40 interplanetary discontinuity marked by a density/dynamic pressure increase with the bow
41 shock launches the full set of forward and reverse fast, slow, and intermediate mode
42 waves into the magnetosheath. The fast forward wave propagates through
43 the magnetosheath and strikes the magnetopause. Here it launches another fast forward
44 mode wave into the magnetosphere and the magnetopause moves inward. The fast
45 reverse wave becomes the new bow shock, which also moves Earthward. These results
46 lead one to expect a step function increase in the solar wind dynamic pressure to initiate 47 abrupt inward motion of the bow shock and magnetopause, as well as an abrupt
48 increase in the magnetospheric magnetic field strength and pressure.
49 There have also been many observational studies concerning the response of
50 the bow shock, magnetopause, and magnetosphere to varying solar wind conditions.
51 Zhang et al. [2009] employed THEMIS observations to time the decelerating inward
52 motion of the bow shock, magnetopause, and transmitted discontinuities that
53 occurred in response to the arrival of an interplanetary shock. Safrankova et al.
54 [2007] showed that the bow shock rebounds following abrupt changes in its location.
55 Koval et al. [2005; 2006], Keika et al. [2009], Andreeva et al. [2011], and Volwerk et
56 al. [2011] presented results from numerical simulations and observations indicating
57 that interplanetary shocks deform upon encountering the bow shock to become
58 concave discontinuities that slow down and engulf the magnetosphere as they pass
59 through the magnetosheath. Nemecek et al. [2011] and Andreeva et al. [2011]
60 presented evidence for the faster antisunward propagation of the transmitted
61 disturbances through the magnetosphere than in the solar wind itself.
62 Results from hybrid code simulations suggest that this simple picture sometimes
63 needs modification. They indicate that hot flow anomalies accompany the interaction of
64 some interplanetary magnetic field (IMF) discontinuities with the bow shock [Omidi and
65 Sibeck, 2007]. Hot flow anomalies lie centered on the discontinuities upstream from the
66 point where they intersect the bow shock. They are bounded by shocks that also extend
67 upstream from the Earth's bow shock, and exhibit greatly heated and deflected solar wind
68 plasmas. Bundles of IMF field lines connected to the bow shock often excavate cavities
69 of depressed magnetic field strength and density bounded by compressional boundaries in 70 the region upstream from the bow shock, but exhibit no shocks, heated plasmas, or
71 deflected flows [Omidi et al., 2009]. The signatures of hot flow anomalies and
72 foreshock cavities have been seen in the magnetosheath, and the corresponding
73 pressure variations may cause large amplitude magnetopause motion and
74 perturbations of the magnetospheric magnetic field [Paschmann et al., 1988; Sibeck
75 et al., 1999].
76 Transient (1-10 min duration) magnetic field and plasma events are common in
77 the vicinity of the dayside magnetopause. They have been attributed to boundary waves
78 driven by solar wind dynamic pressure variations [e.g., Sibeck et al., 1989], unsteady
79 magnetopause merging and the generation of flux transfer events (FTEs) [e.g., Russell
80 and Elphic, 1978], the Kelvin-Helmholtz (KH) instability [e.g., Southwood, 1979] and
81 impulsive plasma penetration [e.g., Lemaire, 1977]. Korotova et al. [2011] showed that
82 one such transient event observed at the magnetopause with FTE characteristics was in
83 fact produced by the interaction of the solar wind and bow shock when a complicated
84 sequence of varying IMF directions and solar wind pressures created significant effects,
85 including inward bow shock and magnetopause motion, compressions of the
86 magnetosphere, and the transient event itself.
87 In this paper we present a multipoint THEMIS case study of a transient event
88 with FTE-like bipolar Bn signatures in the direction normal to the magnetopause
89 observed just inside the pre-noon magnetopause at ~2319 UT on October 15, 2008.
90 Observations indicate that this event was associated with a single transient outward
91 motion of the bow shock. We use a global hybrid code model to explain the observations,
92 demonstrating that an IMF tangential discontinuity launches the pressure pulse that 93 triggers both the transient magnetospheric event and the unusual outward bow shock
94 motion.
95
96 2. Data sets, spacecraft, orbits
97 The five THEMIS spacecraft carry identical instruments. The ESA electrostatic
98 analyzer on each THEMIS spacecraft measures the distribution functions of 0.005 to 25
99 keV ions and 0.005 to 30 keV electrons over 4π steradian, providing accurate high time
100 resolution plasma moments, pitch angle and gyrophase particle distributions as often as
101 each 3s. [McFadden et al., 2008]. The FGM triaxial fluxgate magnetometer measures the
102 background magnetic field and its low frequency fluctuations up to 64 Hz [Auster et al.,
103 2008]. The spacecraft return magnetic field vectors, omnidirectional particle spectra, and
104 plasma moments computed on-board once every 3s throughout their orbit. We compare
105 the THEMIS observations with 0.5s time resolution GOES geosynchronous magnetic
106 field observations [Singer et al., 1996].
107
108 3. Spacecraft observations
109 Figure 1 shows the locations of five THEMIS and GOES 11 and 12 spacecraft from
110 2230 to 2400 UT on October 15, 2008. THEMIS A, D and E moved through the pre-noon
111 magnetosphere outbound from GSM (X, Y, Z) = (6.72, -7.83, -1.91) RE to (7.78, -7.55, -
112 1.76) RE and inbound from (8.59, -1.87, 0.32) RE to (7.92, -1.01, 0.45) RE and from (8.86,
113 -2.28, 0.25) RE to (7.68, -0.75, 0.49) RE, respectively. THEMIS B and C were
114 nominally in the solar wind just outside the pre-noon bow shock, moving from GSM (X,
115 Y, Z) =(4.37, -22.50, -4.01) RE to (5.00, -23.34, -3.66) RE and from (10.63, -16.22, -1.52) 116 RE to (11.12, -16.00, -1.21) RE, respectively. The location and shape of the
117 magnetopause have been taken from the empirical study of Roelof and Sibeck [1993] for
118 the solar wind dynamic pressure of 0.5 nPa and IMF Bz = 0 observed by THEMIS B
119 and C (see below), while the location of the Fairfield [1971] bow shock was scaled to
120 place THEMIS B and C in the solar wind during this interval.
121 From 2313 to 2323 UT on October 15, 2008 all three THEMIS A, D and E
122 spacecraft in the magnetosphere observed a long-duration (~10 min) transient event
123 with magnetic field perturbations characteristic of FTEs. Figure 2 presents the
124 magnetic field and plasma moments in GSM coordinates from 2300 UT to 2340 UT
125 observed by THEMIS A, which was closest to the magnetopause and saw stronger
126 magnetic field and plasma signatures. The transient event at 2319 UT was marked
127 by bipolar (-,+) and (+,-) 5 nT signatures in the Bx and By components, respectively,
128 a positive monopolar variation in the Bz component, and a ~ 13 nT enhancement in
129 the total magnetic field strength. The event is superimposed upon an abrupt
130 increase in the total magnetic field strength at THEMIS A from a minimum value of
131 37 nT before the event to a maximum value of 42 nT after the event.
132 THEMIS D and E observed similar ~13 nT enhancements in the total magnetic field
133 strength (not shown). However, THEMIS A observed a sharper increase in the
134 magnetic field strength and greater perturbations in the Bx and By components,
135 presumably because it was closer to the magnetopause. The transients might be
136 FTEs or waves on the boundary. In either case, they correspond to only slight
137 indentations on the magnetopause. In the ~45 nT magnetic field observed by 138 THEMIS A, a 5 nT perturbation in the direction normal to the nominal
139 magnetopause corresponds to a ~6° indentation in the magnetopause surface.
140 The plasma observations also show transient features typical of magnetospheric
141 FTEs: an increase in density, decrease in temperature, northward (+Vz) and sunward
142 (+Vx) flows, a bipolar (+,-) variation in the Vy component, and a ~ 120 km/sec increase
143 in the total velocity. As described by Korotova et al. [2009], the passage of FTEs
144 displaces the ambient media. Signatures to be observed by a spacecraft within the
145 magnetosphere include inward/outward flow velocities in the direction normal to the
146 nominal magnetopause and flows opposite the direction of event motion (here the Vx and
147 Vz signatures). The northward and sunward flows observed in the magnetosphere
148 outside this event indicate that the event itself was moving southward and antisunward
149 along the magnetopause.
150 As in the previous study of Korotova et al. [2011], we interpret the event in terms
151 of a transient magnetospheric compression and not a burst of reconnection. There are two
152 reasons for this. First, the event is long (~10 min) and was observed deep within the
153 magnetosphere. Such events have previously been attributed to pressure pulses [e.g.,
154 Korotova et al., 2011]. Second, the southward and antisunward motion of the event
155 inferred from perturbations in the flow velocities is inconsistent with the postulated
156 location of THEMIS A northward and dawnward of a tilted subsolar reconnection line for
157 the observed duskward and southward IMF orientation [Korotova et al., 2009].
158 Figure 3 presents GOES 11 and GOES 12 magnetic field observations at early and
159 late post-noon local times, respectively. They show that the magnetospheric compression
160 at the time of the transient event was widespread, consistent with the suggested 161 interpretation of the event [e.g., Korotova, et al., 1997, 2004]. GOES 11 (~1420 LT)
162 observed bipolar magnetic field signatures in the Bx component indicating an indentation
163 while GOES 12 (~1820 LT) does not.
164
165 Table 1. Arrival times of peak magnetospheric compressions in the transient event
166 observed by THEMIS A, D, E and GOES 11/12 and the discontinuity observed by
167 THEMIS B and C
168 ______169 170 Spacecraft Observed Peak Fit Peak 171 Time (UT) Time (UT) 172 ______173 THEMIS B 2319:48 174 THEMIS C 2315:33 175 GOES-11 2318:03 2318:06 176 THEMIS E N.A. 2318:39 177 GOES-12 2319:34 2319:21 178 THEMIS D 2319:15 2319:24 179 THEMIS A 2319:36 2319:40 180 ______181
182 The high time resolution data shown in Figure 4 lets us time the motion of the
183 transient event through the magnetosphere. Only the perturbations are shown, a
184 (different) constant value has been removed from each of the traces. We employ
185 two methods. First, we compare the times of the magnetospheric magnetic field strength
186 maxima observed by GOES-11/12 and THEMIS A and D with the times at the centers of
187 the magnetosheath intervals observed by THEMIS B and C. This method does not work
188 for THEMIS E because this spacecraft observed a more complicated signature with at
189 least two peaks in the total magnetic field strength. The results, shown in the second
190 column of Table 1, indicate event motion from GOES-11 dawnward towards the other 191 spacecraft and duskward to GOES-12. Second, to estimate the errors involved in this
192 method, we also fit higher order polynomials to 5-7 min long intervals encompassing
193 the magnetospheric events and compared the peak values in the fits to the times at
194 the centers of the magnetosheath intervals observed by THEMIS B and C. The
195 results, shown in the third column of Table 2, confirm the sense of propagation and
196 differ by 3-13s from those obtained by the first method. For future reference, note
197 that the magnetic field strength begins to increase at 2301 UT at GOES-11 and near
198 2303 UT at GOES-12.
199 In search of solar wind triggers for the transient magnetospheric compression we
200 inspected ACE, Wind and THEMIS B and C observations for corresponding signatures.
201 ACE was located far upstream at ~ 244 RE during the period of interest and the lag time
202 for the propagation of disturbances to the Earth was about one hour. Figure 5 presents
203 ACE magnetic field and velocity observations from 2200 UT to 2240 UT. ACE observed
204 a discontinuity at ~ 2218 UT, when it was located at GSM (X, Y, Z) = (242.0, -25.6, -
205 18.6) RE. Although the discontinuity was more complicated than a simple rotational or
206 tangential discontinuity, we calculated its normal as the cross-product n=
207 (B1xB2)/(|B1xB2|), where B1 and B2 are the mean magnetic fields before and after the
208 discontinuity. The result of the calculation is presented in Table 2 and indicates that the
209 normal pointed dawnward, southward, and antisunward. Solar wind discontinuities with
210 this orientation should first encounter the post-noon bow shock and then sweep
211 southward and both dawnward and duskward, consistent with the aforementioned
212 observations. Wind lies far (~90 RE) off the Sun-Earth line during the period of interest. 213 Because its observations do not indicate a single pronounced discontinuity and differ
214 strikingly from those at ACE, we use ACE as the appropriate distant upstream monitor.
215 THEMIS B and C were located closer to Earth and provide us with better
216 opportunities to study upstream conditions. Figures 6 and 7 present their observations of
217 the ion flux spectra, magnetic field, and plasma in the vicinity of the bow shock from
218 2300 UT to 2340 UT. The interval can be divided into three very different parts. From
219 2300 to 2314:42 UT at THEMIS C and from 2300 to 2318:12 UT at THEMIS B), the
220 spacecraft were in the quasi-parallel foreshock as indicated by ӨBn < 45°, where ӨBn
221 is the angle between the interplanetary magnetic field and the normal to the local
222 portion of the scaled Fairfield bow shock. The foreshock intervals were characterized
223 by disturbed and slightly negative Bx and Bz components, a positive By component and a
224 total magnetic field strength of ~5 nT. This spiral IMF orientation connected the
225 spacecraft to the pre-noon bow shock. Plasma parameters provide further evidence for
226 increased wave activity during the foreshock. The plasma flow was predominantly
227 antisunward with a velocity of ~320-330 km/sec, density and temperatures oscillated near
228 2 cm-3 and 100 eV, respectively. The dynamic pressure was ~0.3-0.4 nPa. IMF Bz was
229 near zero. As expected on the basis of past work, the velocity within the foreshock
230 observed by THEMIS B and C (Vx and Vtot) was slower than that observed by ACE in
231 the pristine solar wind. Finally, the ion flux energy spectra show the presence of
232 superthermal ions with energies of ~10 keV, a good indicator of the foreshock [Fairfield
233 at el., 1990].
234 The bow shock moved outward during the second interval, from ~ 2314:42 UT at
235 THEMIS C and ~2318:12 UT at THEMIS B for 2-3 min. These are magnetosheath 236 intervals because the density and temperature increased to 8-9 nT and 150-250 eV,
237 respectively, the total velocity decreased to 150-180 km/sec, the velocities were deflected
238 dawnward, and the ion flux energy spectra broadened indicating the presence of 0.01-1
239 keV ions. Although there are sharp increases in density and magnetic field strength on
240 one or both sides of these intervals, these are not the signatures of hot flow anomalies,
241 which are identifiable on the basis of density decreases, sharp flow deflections, and
242 large temperature increases. Because THEMIS C was at least 0.5 RE further from the
243 bow shock along its local normal than THEMIS B was from the bow shock along its
244 local normal, the amplitude of the bow shock motion was at least 0.5 RE.
245 The third interval occurred after the bow shock moved back Earthward past
246 THEMIS C at 2316:24 UT and THEMIS B at 2321:24 UT. The Bx components of the
247 magnetic field became positive, resulting in orthospiral IMF orientations that did not
248 connect the THEMIS spacecraft to the bow shock. Upon exiting the magnetosheath,
249 the spacecraft were initially in a transitional region between the quasi-parallel and
250 quasi-perpendicular foreshock with ӨBn ~ 45°. After 4-5 min, ӨBn increased greatly,
251 indicating that the magnetic field pointed nearly perpendicular to the nominal
252 normal to the bow shock. As a result, wave activity in the magnetic field and plasma
253 parameters stopped and these parameters became steady. The total magnetic field
254 strength and temperatures decreased to 3 nT and 30 eV, respectively, but the density
255 increased up to 3.2 cm-3. The solar wind dynamic pressure increased to 0.5-0.6 nPa.
256 IMF Bz was near zero. The ~10 keV ions disappeared from the energy spectra. There
257 was not much change in the THEMIS plasma flow: the Vz component decreased from ~ -
258 25 to ~ 0 km/s, i.e., the flow became less southward. Contrary to THEMIS, ACE did not 259 observe any change in the Vz component while the Vy component decreased from ~17-
260 20 nT to -5-0 nT after the discontinuity. Discrepancies in the ACE and THEMIS Vy and
261 Vz components could be due to spatial variations in the solar wind.
262 As indicated in Table 1, THEMIS C saw the rotation in the IMF and
263 outward motion of the bow shock before B. It took ~ 4:15 min for the IMF
264 discontinuity to propagate from C (2315:33 UT) to B (2319:48 UT), indicating an
265 IMF discontinuity with a normal very inclined to the Sun-Earth line that is moving
266 slowly dawnward. To determine the orientation of the interplanetary discontinuity
267 from the THEMIS B and C observations, we assumed that it was a tangential
268 discontinuity and calculated its normal as a cross-product. Table 2 presents the
269 results for the normals to the discontinuity observed by THEMIS B and C. As in
270 the case of the ACE observations, they indicate that the normal to the discontinuity
271 pointed dawnward, southward, and antisunward. Differences in the precise
272 orientations of the discontinuities at ACE, THEMIS B, and THEMIS C result from
273 errors, spatial variations in the interplanetary discontinuity, and perturbations
274 associated with disturbed magnetic field directions in the foreshock. The arrow in
275 the bottom left corner of Figure 1 illustrates the normal to the tangential
276 discontinuity calculated from THEMIS B observations. Using the positions of
277 THEMIS B and C, the observed 330 km s-1 solar wind velocity, and the normal for
278 the discontinuity calculated from the THEMIS B observations, we estimate a lag
279 time of ~7 min from THEMIS C to B, somewhat longer than that observed,
280 confirming that although the sense of the normal to the discontinuity is correct, its
281 precise orientation is not very well determined. 282 We should also compare the time when the discontinuity passes THEMIS
283 C to the time when its effects are felt in the magnetosphere. THEMIS C encounters
284 the magnetosheath during an interval centered on 2315:33 UT. Using the normal to
285 the interplanetary magnetic field discontinuity computed from the THEMIS B
286 observations and the observed solar wind velocity, we find that the interplanetary
287 magnetic field discontinuity should have encountered the bow shock at a position
288 directly upstream from the GOES-11 spacecraft at GSM (x, y, z) = (14, 4, 0) RE,
289 some 17 min before it reached THEMIS C, i. e. at 2258 UT. Past studies indicate
290 that IMF features require 4-8 min to cross the magnetosheath [Freeman and
291 Southwood, 1988; Etemadi et al., 1988]. The resulting arrival times of 2302 to 2306
292 UT are slightly later than the time when the magnetospheric magnetic field strength
293 begins to increase at GOES-11, about 2301 UT according to Figure 4.
294 Normals to the bow shock crossings observed by THEMIS B and C oscillate in
295 the manner expected for an antisunward and dawnward propagating wave on the bow
296 shock. We used the coplanarity theorem for estimating shock normals [Lepping and
297 Argentiero, 1971] to determine the orientation of the bow shock at its crossings by
298 THEMIS B and C, n=± (B1xB2) X (B2-B1)/|(B1xB2) X (B2-B1)|, where B1 and B2 are
299 the mean magnetic fields before and after the bow shock crossings. Table 2 presents
300 results from these normal calculations. Figure 1 shows the normals (n1, n2, n3, n4) to the
301 modified bow shock shape as the “bulge” passes THEMIS C and B. The bulges are
302 shown at two times. First (solid curve), when only the outward bulge is present on
303 the bow shock. Second (dashed curve) when an outward bulge is present on the
304 dawn bow shock and an inward bulge (grey curve) on the post-noon magnetopause. 305 The normals are deflected from directions expected for the nominal bow shock
306 and oscillate in the manner expected for an antisunward and southward moving wave on
307 the bow shock boundary, as expected for the derived orientation of the driving
308 interplanetary discontinuity. The similarity of the normals observed by THEMIS B and
309 C (see Table 2) suggest that the shape of the bulge did not change much as it propagated
310 dawnward from THEMIS C to THEMIS B.
311 Knowing that the bow shock moved outward from ~2314:42 to 23:16:24 UT at
312 THEMIS C and from ~2318:12 to 2321:24 UT at THEMIS B we determined that the
313 outward bulge on the bow shock moved dawnward with a velocity of ~251 km/sec.
314 Given the durations of the event at each location, this bulge had a dimension of 4.8 RE
315 in the vicinity of THEMIS C and 7.55 RE in the vicinity of THEMIS B. Since THEMIS
316 C was located ~0.5 RE further from the average position of the bow shock than
317 THEMIS B, we suppose that THEMIS C observed the crest of the bulge while
318 THEMIS B observed its full width.
319 Summarizing the results of this section, the sequence of events observed by THEMIS B
320 and C suggests an explanation in which the bow shock briefly moved outward, perhaps
321 by a transient decrease in the solar wind dynamic pressure applied to the magnetosphere.
322 By contrast, the sequence of event observed by all the spacecraft in the magnetosphere
323 suggests an explanation in which the magnetosphere was briefly compressed, perhaps by
324 a transient increase in the solar wind dynamic pressure. The observations could be
325 reconciled if the IMF discontinuity caused a transient outward motion of the bow shock
326 in addition to launching a transient pressure increase into the magnetosheath. To test this
327 hypothesis, we must examine the predictions of a global hybrid code model. 328 Table 2. Solar Wind Discontinuity and Bow Shock Normals
329 ______330 Bow Shock Discontinuity 331 Spacecraft Representative Times nx ny nz nx ny nz 332 ______333
334 ACE 2216:13 - 2219:25 -0.41 -0.36 -0.89
335 THEMIS C 2313:55 - 2316:32 -0.23 -0.73 -0.64
336 THEMIS B 2317:38 - 2322:05 -0.37 -0.59 -0.71
337 THEMIS C 2314:28 - 2314:58 0.40 -0.72 -0.56 (n1)
338 THEMIS C 2316:05 - 2316:38 -0.89 -0.31 -0.34 (n2)
339 THEMIS B 2317:59 - 2318:32 0.48 -0.86 -0.38 (n3)
340 THEMIS B 2321:15 - 2321:45 -0.88 -0.47 0.03 (n4)
341 ______
342
343 4. Description of global hybrid code model.
344 We examine output from a global hybrid model similar to that presented by
345 Omidi and Sibeck [2007] in which ions are treated kinetically via particle-in-cell methods
346 and electrons form a massless fluid. The simulation plane corresponds to the noon-
347 midnight meridion plane with Y pointing northward (see Figure 8). Solar wind plasma
348 enters the simulation domain from the left boundary and leaves through the three
349 remaining boundaries. Although the magnetosphere is 7 times smaller than that of the
350 Earth, the model still captures the relevant physics. The simulation retains all three
351 components of the electromagnetic fields and plasma flows. The solar wind Alfvén
352 Mach number is set to 12, ion and electron betas are set to 0.3. Cell sizes in the 353 simulation are 1 c/ωpi where c is the speed of light and ωpi is the proton plasma frequency,
354 and the resistive scale length is 0.3 c/ωpi. The simulation box extends to 2000 c/ωpi in X
355 and Y directions respectively with the Earth’s dipole centered at X = 1500 and Y = 1250.
356 Prior to the arrival of the tangential discontinuity, the IMF lies in the X-Y (meridional)
357 plane, whereas it rotates at the discontinuity to develop a duskward Z component. There
358 is no change in the magnetic field strength, density, velocity, or temperature across the
359 discontinuity.
360 Figure 8 shows a color intensity plot of the predicted density normalized to the
361 solar wind density in a region centered on the southern foreshock and the bow shock. We
362 wish to call attention to two features: (1) an outward motion of the bow shock following
363 the passage of the tangential discontinuity and (2) a front marked by a transient increase
364 in the density (pressure) launched into the magnetosheath.
365 Concerning the first topic, we note that a highly turbulent foreshock lies
366 upstream from the quasi-parallel bow shock at locations antisunward (to the right) of the
367 tangential discontinuity. By contrast, the solar wind is in a pristine condition upstream
368 from the quasi-perpendicular bow shock at locations sunward (to the left) of the
369 tangential discontinuity. As indicated by the density contours in Figure 8, the passage of
370 the discontinuity causes the bow shock to move outward from a position nearer Earth in
371 the quasi-parallel configuration to one further from Earth in the quasi-perpendicular
372 configuration. These results are consistent with results from the simulation reported by
373 Thomas and Winske [1990], a observations from Venus reported by Zhang et al. [1991],
374 and observations of the terrestrial bow shock reported in Figure 5 of Verigin et al.
375 2001]. Because the bow shock lies along the locus of points where the components of the 376 solar wind velocity and magnetosheath fast mode speed normal to the bow shock balance,
377 and fast mode speeds are greater perpendicular than parallel to magnetosheath magnetic
378 fields, theory predicts outward bow shock motion for a transition from quasi-parallel to
379 intermediate or quasi-perpendicular shocks.
380 The actual tangential discontinuity on October 15, 2008 was accompanied by an
381 increase in the solar wind density and therefore dynamic pressure, as indicated by the
382 jumps in density from times before the magnetosheath encounters to times after the
383 magnetosheath encounters in Figures 6 and 7. This increase in the solar wind dynamic
384 pressure should push the bow shock (and magnetopause) inward, not outward. The
385 actual motion of the bow shock must therefore be the sum of the outward motion
386 associated with the rotation in the IMF direction and inward motion associated with the
387 step function increase in the solar wind dynamic pressure. The outward motion of the
388 bow shock can therefore be transient.
389 To simulate the sequence of events that would be observed by a spacecraft
390 initially just upstream from the quasi-parallel bow shock during the passage of the
391 tangential discontinuity, we take a cut of the plasma and magnetic field observations
392 along the line labeled “L” in Figure 8 that grazes the bow shock. Figure 9 shows that the
393 spacecraft first observes the turbulent quasi-parallel foreshock, briefly enters the
394 magnetosheath, and then reenters the solar wind upstream from the quasi-perpendicular
395 bow shock. This is very similar to the scenarios seen by THEMIS B and C, as shown in
396 Figures 6 and 7.
397 Concerning the second topic, we note that the simulation indicates the
398 transmission of a transient density increase into the magnetosheath. To simulate the 399 sequence of events that would be observed by a spacecraft initially in the magnetosheath,
400 we take a cut of the plasma and magnetic field observations across this increase, i.e.
401 along the line labeled “L1” in Figure 8. Figure 10 shows that the spacecraft observes a
402 transient increase in the density and dynamic pressure, but no significant change in the
403 total velocity, temperature, or magnetic field strength as the density front passes by. This
404 transient increase in density must be added to the step function increase in the solar wind
405 density observed on October 15, 2008, resulting in a transient compression of the
406 magnetosphere superimposed upon a step function increase in magnetospheric magnetic
407 field strengths. Inspection of Figures 2 and 3 shows that this is precisely the case for the
408 THEMIS A and GOES 11 magnetospheric magnetic field strength observations.
409 410 5. Conclusions
411 We presented a multipoint THEMIS case study of a transient event observed
412 inside the pre-noon magnetopause at 2319 UT on October 15, 2008. Multipoint
413 observations indicate a global compression of the magnetosphere corresponding to a
414 transient outward bow shock motion. We used results from a global hybrid code model
415 for the interaction of an IMF tangential discontinuity with the bow shock to reconcile the
416 observations. The arrival of a discontinuity that transforms the bow shock from quasi-
417 parallel to quasi-perpendicular launches a narrow density front into the magnetosheath
418 that briefly compresses the magnetosphere when it strikes the magnetopause. The same
419 discontinuity initiates outward bow shock motion and contributes to an additional
420 compression of the magnetospheric magnetic field.
421
422 Acknowledgements. Work at GSFC was supported by the THEMIS project, while 423 work by G. I. K. at the University of Maryland was supported by a grant from
424 NASA/GSFC NNX09AV52G.
425
426 References
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548 Figure Captions
549 Fig.1. Locations of THEMIS A, B, C, D, E and GOES 11 and 12 in the GSM X-Y plane
550 from 2230 UT to 2400 UT on October 15, 2008. The bulges are shown at two times.
551 First (solid curve), when only the outward bulge is present on the bow shock.
552 Second (dashed curve) when an outward bulge is present on the dawn bow shock
553 and an inward bulge (grey curve) on the post-noon magnetopause. Normals (n1, n2,
554 n3, n4) to the modified bow shock (BS) shape are shown as the “bulge” passes THEMIS
555 C and B. The curve labeled MP shows the corresponding inferred inward deformations of
556 the magnetopause. The arrow in the bottom left corner of the figure illustrates the normal
557 to the tangential discontinuity observed by THEMIS B.
558 559 Fig.2. THEMIS A plasma and magnetic field observations from 2300 UT to 2340 UT on
560 October 15, 2008. From top to bottom, the panels show the Bx, By, Bz components of
561 magnetic field in GSM coordinates and total magnetic field strength, the ion density, the
562 velocities in GSM coordinates, the ion temperatures perpendicular and parallel to
563 magnetic field. Dashed lines bound the transient event.
564
565 Fig. 3. GOES-11 and -12 magnetic field observations in GSM coordinates from 2300 UT
566 to 2340 UT on October 15, 2008. Arrows show a compression of the magnetosphere.
567
568 Fig. 4. Variations in the total magnetic field strength observed by GOES-11 and -
569 12, THEMIS A and D from 2300 to 2330 UT on October 15, 2008. A constant value
570 has been subtracted from each trace so that they can be graphed on the same scale.
571
572 Fig. 5 ACE observations of the magnetic field and velocity in GSM coordinates from
573 2200 UT to 2240 UT on October 15, 2008. The arrow indicates a discontinuity.
574
575 Fig. 6. THEMIS C observations of ion energy spectra, plasma and magnetic field from
576 2300 UT to 2340 UT on October 15, 2008. From top to bottom, the panels show the flux
577 spectrogram for ions in the range of energies from 2 eV to 25 keV (ESA), ӨBn, the angle
578 between the magnetic field and the local bow shock normal, dynamic pressure, Bx,
579 By, Bz components of magnetic field in GSM coordinates and total magnetic field,
580 the ion density, the velocities in GSM coordinates, the ion temperatures
581 perpendicular and parallel to magnetic field. The spacecraft began the interval in 582 the quasi-parallel foreshock (ӨBn < 45°). Two vertical dashed lines bound a brief
583 period in the magnetosheath. Upon exiting the magnetosheath, the spacecraft was
584 in a transitional region between the quasi-parallel and quasi-perpendicular
585 foreshock (ӨBn ~ 45°). The third vertical dashed line marks the transition to the
586 quasi-perpendicular bow shock (ӨBn > 45°).
587
588 Fig. 7. The same as for Fig.6 except for THEMIS B observations.
589
590 Fig. 8. Color intensity plot of density in the run for a portion of the simulation box (noon-
591 midnight meridian plane) containing the dayside and post-noon bow shock. The density
592 is normalized to the solar wind density, X points antisunward and Y points northward.
593
594 Fig.9. Snapshots of ion Vx and Vy velocities, magnetic field strength, and density along
595 the cut labeled “L” in Figure 8. Velocities are normalized to the Alfvén speed in the solar
596 wind while the magnetic field and density are normalized to their corresponding values in
597 the solar wind.
598
599 Fig.10. Snapshots of magnetic field strength, temperature, magnitude of the ion velocity,
600 density and dynamic pressure along the cut labeled “L1” in Figure 8. Velocity is
601 normalized to the Alfvén speed in the solar wind while the magnetic field and density are
602 normalized to their corresponding values in the solar wind.
603
604 605
606
607
608
609 Fig.1. Locations of THEMIS A, B, C, D, E and GOES 11 and 12 in the GSM X-Y plane
610 from 2230 UT to 2400 UT on October 15, 2008. The bulges are shown at two times.
611 First (solid curve), when only the outward bulge is present on the bow shock.
612 Second (dashed curve) when an outward bulge is present on the dawn bow shock
613 and an inward bulge (grey curve) on the post-noon magnetopause. Normals (n1, n2,
614 n3, n4) to the modified bow shock (BS) shape are shown as the “bulge” passes THEMIS
615 C and B. The curve labeled MP shows the corresponding inferred inward deformations of
616 the magnetopause. The arrow in the bottom left corner of the figure illustrates the normal
617 to the tangential discontinuity observed by THEMIS B.
618
619 620
621 Fig.2. THEMIS A plasma and magnetic field observations from 2300 UT to 2340 UT on
622 October 15, 2008. From top to bottom, the panels show the Bx, By, Bz components of
623 magnetic field in GSM coordinates and total magnetic field strength, the ion density, the
624 velocities in GSM coordinates, the ion temperatures perpendicular and parallel to
625 magnetic field. Dashed lines bound the transient event. 626
627 Fig.3. GOES 11 and GOES 12 magnetic field observations in GSM coordinates from
628 2300 UT to 2340 UT on October 15, 2008. Arrows show a compression of the
629 magnetosphere.
630 631
632
633 Fig. 4. Variations in the total magnetic field strength observed by GOES-11 and -
634 12, THEMIS A and D from 2300 to 2330 UT on October 15, 2008. A constant value
635 has been subtracted from each trace so that they can be graphed on the same scale.
636
637
638 639
640
641 Fig. 5 ACE observations of the magnetic field and velocity in GSM coordinates from
642 2200 UT to 2240 UT on October 15, 2008. The arrow indicates a discontinuity.
643
644
645 646
647
648 649
650 Fig. 6. THEMIS C observations of ion energy spectra, plasma and magnetic field from
651 2300 UT to 2340 UT on October 15, 2008. From top to bottom, the panels show the flux
652 spectrogram for ions in the range of energies from 2 eV to 25 keV (ESA), ӨBn, the angle
653 between the magnetic field and the local bow shock normal, dynamic pressure, Bx,
654 By, Bz components of magnetic field in GSM coordinates and total magnetic field,
655 the ion density, the velocities in GSM coordinates, the ion temperatures
656 perpendicular and parallel to magnetic field. The spacecraft began the interval in
657 the quasi-parallel foreshock (ӨBn < 45°). Two vertical dashed lines bound a brief
658 period in the magnetosheath. Upon exiting the magnetosheath, the spacecraft was
659 in a transitional region between the quasi-parallel and quasi-perpendicular
660 foreshock (ӨBn ~ 45°). The third vertical dashed line marks the transition to the
661 quasi-perpendicular bow shock (ӨBn > 45°).
662
663
664
665
666
667 668
669 670 Fig. 7. The same as for Fig. 6 except for THEMIS B observations.
671 672 673 674 675
676 677 678 Fig. 8. Color intensity plot of density in the run for a portion of the simulation box (noon-
679 midnight meridian plane) containing the dayside and post-noon bow shock. The density
680 is normalized to the solar wind density, X points antisunward and Y points northward.
681 682
683 Fig.9. Snapshots of ion Vx and Vy velocities, magnetic field strength, and density along
684 the cut labeled “L” in Figure 8. Velocities are normalized to the Alfvén speed in the solar
685 wind while the magnetic field and density are normalized to their corresponding values in
686 the solar wind.
687 688
689 Fig.10. Snapshots of magnetic field strength, temperature, magnitude of the ion velocity,
690 density and dynamic pressure along the cut labeled “L1” in Figure 8. Velocity is
691 normalized to the Alfvén speed in the solar wind while the magnetic field and density are
692 normalized to their corresponding values in the solar wind.
693
694
695
696 697
698
699
700
701